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Guidelines for Monitoring for Landfill Gas at and Near Former Dumps

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Guidelines for Monitoring for Landfill Gas at and Near Former Dumps Guidelines for Monitoring for Landfill Gas at and Near Former Dumps Remediation Division Voluntary Investigation and Cleanup, Emergency Response, Superfund, and Resource Conservation and Recovery Act Corrective Action Programs Introduction This document was developed in cooperation with the Minnesota Department of Health and staff at the Minnesota Pollution Control Agency (MPCA) Closed Landfill Programs to provide guidelines for the monitoring of methane associated with landfill gas (LFG) generated from abandoned unpermitted dumps that may pose risk to existing or proposed occupied structures. Non-methane organic compounds such as volatile organic compounds (VOCs) associated with historical releases at dumps (e.g., trichloroethylene, benzene, etc.) also may present in LFG, although the focus of this guidance principally concerns the monitoring of and response to methane hazards. Abandoned dumps are not included in the state legislation which provides for the cleanup of qualified landfills. The releases associated with abandoned dumps are commonly addressed by voluntary or responsible parties with the MPCA Superfund or Voluntary Investigation and Cleanup (VIC) Programs. Detailed guidance regarding the investigation, mitigation or long term management of subsurface landfill gas is beyond the scope of this document. Generation and migration of landfill gas Landfill gas (LFG) generated from landfills and abandoned mixed municipal dumps consists primarily of methane, carbon dioxide and smaller fractions of numerous other gases including nitrogen, oxygen and ammonia produced by the biodegradation of organic matter. This biodegradation is the result of the activity of microorganisms that are found naturally occurring in both wastes and soils. Methane can also be generated by naturally occurring buried organic deposits (e.g., peaty sediments) and can be difficult to distinguish from other methane sources. Although the processes by which LFG is generated are similar at all dumps, considerable variability will exist between dumps in the amount of gas generated, the composition of the gas and the gas generation rate. Generally, methane generation is divided into four distinct phases: Phase I Upon initial placement of the waste, an aerobic phase develops characterized by rapid oxygen depletion as a result of increased microbial activity. This process can last for several months in larger dumps. Phase II At the end of Phase I, oxygen is depleted and anaerobic microbial activity is initiated, signaling the beginning of Phase II. During the anaerobic phase, leachate is produced, acidity is increased and there is a steady increase in hydrogen and carbon dioxide production. Phase III The start of Phase III is initiated by the production of methane gas. This is an anaerobic phase characterized by an accelerated increase in methane production with corresponding decreases in the levels of carbon dioxide, hydrogen, nitrogen, and acidity. Phase IV The final phase is characterized by a long period of methane production and waste degradation as methanogenic microorganisms reach steady state populations and gas constituent concentrations stabilize. Methane generation may last from several years to decades. The length of the four phases of methane generation will vary considerably for different dump settings depending upon the amount of waste present, the composition of the waste material, moisture levels, temperature, and operational practices of the former dump. c-rem3-04 November 2011 Minnesota Pollution Control Agency • 520 Lafayette Rd. N., St. Paul, MN 55155-4194 • www.pca.state.mn.us 651-296-6300 • 800-657-3864 • TTY 651-282-5332 or 800-657-3864 • Available in alternative formats One of the most significant factors controlling waste degradation is moisture content. The mummified conditions observed at many dumps and landfills, as evidenced by the readability of old newspapers is directly related to a relative lack of moisture needed to promote degradation. Capping of a dump or landfill to reduce moisture infiltration will therefore, also reduce the rate of gas production and waste degradation. The presence of a cap also may essentially eliminate vertical LFG escape and promote lateral LFG migration. Waste composition affects both the methane generation rate and the total amount of methane produced. Wastes containing higher biodegradable organic content, such as food waste, wood and paper, will produce more methane than more inert material such as concrete, bricks, plastic and glass. Typical municipal wastes products found in former dumps such as food and yard debris contain high amounts of biodegradable material that can result in high levels of methane generation. Temperature also has an effect upon microbial activity. Generally, higher fill temperatures result in higher rates of methane production. The optimal temperature range for methane generation is between about 95ºF to 120ºF. Methane generation can be nonexistent at temperatures below 50ºF, which may be an important consideration in Minnesota. Factors affecting LFG emissions and migration LFG emissions to the atmosphere can occur via vertical migration through the surface cover of dump and/or at perimeter locations around the dump through a combination of lateral and vertical migration. LFG migrates from areas of high pressure to areas of low pressure, driven by subsurface and atmospheric pressure gradients. High pressure conditions are created within the waste mass of a dump when methane gas generation is taking place. Meteorological conditions can also affect the migration of LFG. Relative changes in barometric pressure may accentuate LFG pressure gradients in the subsurface around dump sites resulting in an increase in vertical and lateral gas migration, and a concomitant increase in the potential for vertical escape of emissions to the atmosphere. LFG migration can be expected to occur laterally and vertically along preferential pathways where higher permeable native soils or fill are present or along buried utility corridors backfilled with coarser or aggregate than surrounding soils. Lateral LFG migration can be enhanced during winter months as vertical gas escape routes are limited by thick frozen soil columns typical of Minnesota winters. Similarly, at dump sites where impermeable covers, engineered caps or asphalt surfaces have been constructed the potential for lateral migration of LFG beyond the boundaries of the dump site can be enhanced although the resulting decrease of infiltrating moisture from impermeable caps should support lower organic degradation rates. Environmental concerns of LFG Environmental impacts of LFG and of methane gas in particular, can be separated into three main categories: imminent physical hazards, inhalation risks, and ecological impacts. LFG hazards The principal hazards associated with LFG are explosion and fire. There is a risk that methane can migrate laterally and vertically through the unsaturated soil column and collect in enclosed or confined spaces where a spark or other ignition source can trigger an explosion or fire. Methane presents an explosive hazard at concentrations between 5 percent and 15 percent by volume, in air. The lower and upper levels of the range of combustible gas concentration within which explosion may occur are defined for a specific combustible gas, and are known, respectively, as the Lower Explosion Limit (LEL) and the Upper Explosion Limit (UEL). The LEL for methane corresponds to five percent methane by volume in air, which is equivalent to a relative concentration of 50,000 parts per million (ppm) methane by volume. The UEL for methane is 15 percent methane by volume in air. Inhalation risks Risks associated with LFG inhalation can include both short term and long term exposure risks. Both of the two main LFG components, methane and carbon dioxide, are colorless and odorless, and have the capability to displace oxygen, which can result in conditions with the potential for asphyxiation. The accumulation of such lethal levels is especially a concern in confined spaces, such as underground utility structures and trenches, and within enclosed spaces and basements in buildings located at or adjacent to a dump or a landfill. The short term effects of LFG inhalation can include headaches and irritability. Hydrogen sulfide, an odorous gas, which can be found at toxic levels in LFG, is an irritant to the eyes and respiratory system and can also be an asphyxiant. Long term inhalation risks may be associated with VOCs associated with LFG. Guidelines for Monitoring Landfill Gas at Near Former Dumps • c-rem3-04 • November 2011 Page 2 of 10 Ecological risks LFG can stress or even kill plant life by displacing oxygen within the soil near the roots of plants. Crop damage can occur at farms near landfills. Additionally, although present in much smaller concentrations than methane and carbon dioxide, some non-methanogenic VOCs can contribute to the formation of ozone, a gas which, in addition to having deleterious respiratory effects, also can reduce plant growth and contribute to vegetation damage. Landfill Gas Monitoring and Analysis Preliminary subsurface methane surveys Gas surveys should be conducted at abandoned dumps to assess methane risk levels to nearby or proposed structures. Due to the potential for changes in gas concentrations that may occur seasonally along with changes in moisture, ground temperature and frost conditions, LFG monitoring should be conducted at least three to four times per year. To the degree
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